Lung vascular injury leading to protein-rich edema formation is a hallmark of ALI and respiratory failure in critically ill patients. Little is known about the mechanisms of lung vascular endothelial regeneration following vascular injury. However, with an ever-growing understanding of Endothelial Progenitor Cells (EPCs) and our ability to identify and obtain them in sufficient numbers from induced Pluripotent Stem Cell-derived Endothelial Progenitor Cells (iPSC) or through direct reprogramming of somatic cells (fibroblasts), it is feasible to address the role EPCs in promoting vascular regeneration and to define the mechanism of vascular regeneration. In addition, we have developed a mouse reporter model employing the tamoxifen- inducible endothelial specific Scl-Cre (End-Scl-Cre-ER) which enables the rigorous tracing of endothelial lineage following endothelial injury. Using this approach we are also in the position of addressing mechanisms of intrinsic endothelial regeneration and restoration of lung vascular integrity. This proposal focuses on restoration of the injured lung endothelium by endogenous cells as well as transplantation of exogenous regenerative EPCs. In Aim 1 we will investigate the efficacy and fate of transplanted iPSC-EPCs (induced pluripotent stem cells-derived endothelial progenitor cells) following lung vascular injury. We will test the hypothesis that iPSC-EPC transplantation prevents pulmonary edema and improves survival after lung injury by acutely restoring barrier function as well as through engraftment into the lung microvasculature, and thus restores lung fluid balance. In Aim 2 we will study the lung vascular regenerative potential of mouse fibroblasts that have undergone lineage conversion into endothelial cells (Fib-EPCs). We will test the hypothesis that adult fibroblasts converted directly into functional proliferative EPCs using a novel microRNA strategy restore lung endothelial barrier function and fluid balance and prevent pulmonary edema. In Aim 3 we will identify through endothelial lineage tracing populations of endogenous reparative cells and determine whether their activation promotes lung vascular regeneration and restoration of lung fluid balance. We will test the hypothesis that activation and proliferation of endogenous reparative endothelial cells restores lung endothelial barrier and fluid balance following vascular injury. The above studies will provide the essential frame-work needed to develop novel therapies for endothelial regeneration and recovery after lung vascular injury.